Oregon´s provisional government passed the first militia law on July 5, 1843. It authorized the forming of a battalion of mounted riflemen, composed of the male inhabitants of Oregon between the ages of 16 and 60 who wish to be recognized as citizens by the new government. The law allowed for the annual election of officers who would inspect their volunteer troops once a year and could assemble them at any time in the case of invasion or local insurrection. The troops were also subject to call, with governmental consent, by any authorized agent of the United States government. In reality, while officers were commissioned, militia units were never formed due to the political climate in Oregon. The large population of French Canadians and Hudson´s Bay officials and employees did not recognize the government and distrusted the formation of any organized military unit in the disputed territory.

A second military bill passed on March 9, 1844. It called for the organization of militia companies to combat hostile Indians. The bill led to the formation of the Oregon Rangers on March 11, 1844, a fifteen man force which drilled at the Oregon Institute but saw no action. That summer, the legislature again amended the militia law. Worries over defense continued, and in 1845 a legislative committee made up of one representative from each county was appointed to draft a comprehensive bill "for the protection of this colony." This would include the building of blockhouses and arms magazines and further revising of the militia law. The decision to form a coalition government with British citizens and limited funds doomed these plans.

The situation did not change after the 1846 treaty which gave the United States territorial jurisdiction over Oregon. No organized militia existed until December 1847 and the Cayuse attack on the Whitman mission at Waiilatpu. On December 8, the legislature authorized the formation of a 50 man rifle company to relieve the mission and protect survivors. Forty-five men quickly volunteered and left Oregon City on boats for Fort Vancouver the next day, even as the legislature voted to raise a regiment for a campaign against the Indians. On December 28, the office of adjutant general was created to oversee administrative and logistical control over the troops.

Territorial status brought to Oregon arms for its arsenals, money to pay the volunteers who fought the Cayuses, and federal troops to assist in fighting future Indians wars. Bills passed by the Oregon legislature in 1854, 1855, and 1856 reorganized the militia and allowed occupational and religious exemptions from military service. The state constitution passed in 1857 summarized the existing laws to that time. Its provisions defined enlistment in the militia, listed exemptions, made the governor ex-officio commander-in-chief of the military and naval forces of the state, allowed for the executive appointment of an adjutant general and general staff officers, and authorized the legislature to provide necessary rules and regulations for governing the militia.

The military code adopted in 1862 formally gave the governor´s office its powers over the militia. It listed the members of the general staff and formed a Board of Military Auditors to review volunteer companies´ claims for ammunition and other stores. The legislature amended the law two years later by expanding the duties of the adjutant general. Several motions to repeal the military code were defeated in the special session of 1865, and the legislators authorized a military fund be set up to offer cash payments to volunteers as an inducement to drill, including rewards to the best drilled companies and the paying of expenses for companies called to parade.

The legislature abolished the adjutant general´s office in 1870 and transferred its duties to the secretary of state. However, the position returned when another military code passed in 1887. It authorized the governor to appoint an adjutant general, other chief officers of the general staff, his own staff, and the State Military Board, the latter to act as advisors to him on military affairs. The law also did away with the Board of Military Auditors and designated the active militia as the Oregon National Guard and the inactive militia as the Oregon Reserve Militia.

A comprehensive 1895 law directed that all general officers be selected by the governor, with the consent of the state senate, and serve a four year term. The general staff would consist of the adjutant general as chief of staff (a position still held today) and eight other designated generals appointed by the governor and holding office at his pleasure. The legislation named to the military board the adjutant general, inspector general, judge advocate general, brigadier general, and surgeon general. In 1901 and 1905, laws amended the required compositions of both the general staff and military board. Another reform abolished the military board in 1909 and gave its advisory duties to the general staff. And in 1915, a bill passed that organized all National Guard units in accordance with United States Army rules and regulations. This meant the governor temporarily had no military staff under the new table of organization.

A state naval militia was established in 1911 and placed under the administration of a board consisting of five commissioned naval officers appointed by the governor. The board existed four years. It was replaced by a naval staff, which included the adjutant general as chairman and two commissioned naval militia officers appointed by the governor. A more comprehensive naval code went into effect in 1917. It allowed the militia to conform to a 1914 naval department act detailing accepted federal rules and regulations governing state naval militias. The federal government loaned the state the U.S.S. Boston, a third-class cruiser, as a militia training ship, and detailed an officer and several chief petty officers from the regular navy as inspectors-instructors. The cruiser U.S.S. Marblehead and torpedo boat destroyer Goldsborough were also made available for militia use. Oregon´s naval militia went into active federal service in 1917. It was subsequently placed into the United State naval reserves and ceased to function as a state unit.

The legislature adopted another military code on the eve of World War I. It named the senior line officer to succeed as commander-in-chief in the event state officers in the regular line of succession were not available. The law also formed a state administrative staff from members of the adjutant general, inspector general, judge advocate general, quartermaster corps, medical, and ordinance departments. Four years later the code was revised to conform to the National Defense Act of 1920. It authorized the governor to appoint, subject to approval by the federal government, a National Guard officer to act as property and disbursing officer for the United States in Oregon and as state property officer.

Two agencies involved with state World War I veterans were created in 1919. A committee to provide designs for medals included the governor, secretary of state, treasurer, state librarian, and adjutant general. It functioned until 1929. The Overseas Welcome Commission, set up to welcome returning resident veterans disembarking in New York City consisted of five executive appointees and was abolished in 1927. In 1923, the state accepted the battleship U.S.S. Oregon and designated the general staff to supervise its maintenance. The legislature changed this two years later and authorized the governor to appoint a five-member commission to assume those duties. After the ship was returned to the federal government during World War II for the reclaiming of its metal, the commission was placed in charge of administering a marine museum which included relics from the ship. The legislature abolished the commission in 1957.

No major revisions in the military code occurred during the rest of the 1920s and 1930s. Laws passed dealt mainly with commissioned officers, although one 1931 bill recognized the service of Indian war volunteers upon accepted proof of participation. This changed with the coming of World War II. President Franklin D. Roosevelt declared a limited national emergency after Germany´s invasion of Poland in September 1939. Oregon responded by being the first in the nation to attain its authorized increase in national guard manpower: over 900 men in less than a week. Roosevelt mobilized most of the Oregon guardsmen by executive order in August 1940, when he named the 41st Division as one of four National Guard divisions to be called up. By 1943, over 6,000 men from the Oregon national guard and guard reserves entered federal service.

Oregon took several measures to bolster its defenses during the war. The legislature formed the Oregon State Guard the day after the attack on Pearl Harbor to assume the local role of departing guard troops. Aircraft observers had already been recruited and observation posts established throughout western Oregon by the fall of 1941. After war was declared, most of these 500 posts were manned 24 hours a day, until placed on a reserve status by the army in 1943. Bills passed in 1941 and 1945 to construct, equip, and furnish armories. The federal Office of Civilian Defense helped with the organizing of a civil defense network within Oregon, the structure for which was already in place due to the appointment of a state defense council by the governor in 1941. All war-connected services other than protection were merged into one division known as the Civilian War Services in July 1942.

The next year the legislature reorganized the state guard and passed a civil defense act which officially created the Oregon State Defense Council. Headed by the governor, the council supervised and coordinated civil defense activities in the state. Also put into operation were an Internal Security Section under the direction of the governor, a state Information and Public Relations Division to promote civil defense activities, and a Rumor and Propaganda Division whose task was to receive and track down rumors to check on their validity and thus undermine and minimize enemy propaganda efforts in Oregon.

The end of the war brought a brief return to pre-war conditions. National Guard troops returned from federal service in 1945. The state defense council stopped functioning in 1947, and the state guard was deactivated at the end of June 1948. However, similar operations were revived on the eve of the Korean conflict. The legislature passed the Oregon Civil Defense Act of 1949 as a response to world tensions and the need for a local defense force in the event of natural or man-made disasters. The act created the Civil Defense Agency, the Civil Defense Advisory Council, and authorized a number of mobile reserve battalions to be called to duty upon orders of the governor. Another 1949 law declared an emergency and formed a national guard reserve unit. The war caused all tactical and air National Guard units in Oregon to be activated into federal service in 1950 and 1951. They were returned to state control by the end of 1953, the same year the legislature officially abolished the state guard.

The adjutant general´s office had acted as a de facto military department since its inception. Sometimes referred to as the military department, by the mid-1950s it was divided into four operating units: administration and personnel, operations and training, purchasing and disbursing, and maintenance. The latter section maintained camps, rifle ranges, and other installations scattered throughout the state. Located at Salem, civilian personnel were appointed by the adjutant general and all assigned military personnel were detailed to it by the adjutant general. Office functions included liaison with the federal government in all military matters affecting the state, keeper of all national guard and state guard personnel records, custodian of relics and memorabilia, quartermaster-general in time of peace, custodian of all state military camps and installations, and overseer of administration and training of the National Guard and storage and distribution of its equipment.

The legislature recognized these operations when they reorganized the militia in 1961 and officially created the Military Department. The department retained all of the functions previously done by the adjutant general´s office. Clauses in the law dealt with administration, personnel, leases and agreements on the use of armories, and disposition of receipts from federal and state monies, including the creation of a revolving fund.

The department was organized into a three-tier operation by 1966. The governor remained in titular control, with a personal staff and the Military Council to advise him. The adjutant general headed the department, and an administrative assistant and the State Armory Advisory Board reported directly to him. Everyday activities were divided among a primary staff with five broad management functions (administration, operations and training, U. S. property and fiscal office, comptroller, and installations) and a special staff with five specialized departments (public information, state maintenance, military support and plans, air national guard base detachment, and army technician personnel). These were overseen by the assistant adjutant general. In addition, a United States Army Advisory Group was assigned to advise and assist in the training of all guard units in the state.

The 1961 militia law remains in effect today. Amendments passed since then include conditions for the appointment of assistant adjutant generals, bills on the regulation of funds, an act making the department the official depository of historical items, a law on grants and donations for the Oregon National Guard Military Museum and Resource Center, and an act on property loss incidental to the activities of the national guard. The department administers 38 armories, two army aviation support facilities, three air base complexes, two camps, and three training sites. Each community with a sufficient population to support guard activities has a guard unit assigned to it.

The department today retains the adjutant general as director, with two assistant adjutants general, one each from the army and air National Guard, next in the line of command.